Apparatus for ultrasonic etching of polymeric printing plates

ABSTRACT

THE INVENTION DISCLOSED IS AN APPARATUS FOR ULTRASONIC ETCHING OF POLYMERIC PRINTING PLATES. THE APPARATUS INCLUDES A TANK CONTAINING A LIQUID BATH AND HAVING ULTRASONIC ENERGY MEANS FOR ENERGIZING THE BATH. THE ULTRASONIC MEANS ARE DISPOSED ALONG ONE INNER VERTICAL WALL OF THE TANK.

Nov. 14, 1972 F. A. WESSELLS ETAL 3,702,795 APPARATUS FOR ULTRASONIC EICHING 0F POLYMERIC PRINTING PLATES Filed Dec. 24, 1970 3 Sheets-Sheet 1 INVENTORS FORREST A. WESSELLS DONALD P. GUSH IY V ATTORNEY Nov.- 14, 1972 F. A. WESSELLS ET AL 3,702,795

APPARATUS FOR ULTRASONIC ETCHING 0F POLYMERIC PRINTING PLATES Filed Dec. 24, 1970 3 Sheets-Sheet 2 AME MIR. SUPPLY 72 f g k 0 F76. 3 a 1x. Qva

TO DRAIN U6 "VI em FORREST l. WEWLLS 8y DONALD P. 60:0

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Nov. 14, 1972 F. A. WESSELLS ETAL 3,702,795

APPARATUS FOR ULTRASONIC ETCHING OF POLYMERIC PRINTING PLATES 3 Sheets-Sheet 5 Filed Dec. 24, 1970 AIFL i \lil II II Iii Illiil I! FIG. 4

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IIIIII AT TORNEY United States Patent 3,702,795 APPARATUS FOR ULTRASONIC ETCHING 0F POLYMERIC PRINTING PLATES Forrest A. Wessells, Baltimore, and Donald P. Gush, Hyattsville, Md., assignors to W. R. Grace & C0., New York, N.Y.

Continuation-impart of application Ser. No. 764,849, Oct. 3, 1968, which is a continuation-in-part of application Ser. No. 707,299, Feb. 21, 1968, both now abandoned. This application Dec. 24, 1970, Ser. No. 101,352

Int. Cl. C23g 1/06; Hk 3/00 US. Cl. 156-345 6 Claims ABSTRACT OF THE lDlSCLOSURE The invention disclosed is an apparatus for ultrasonic etching of polymeric printing plates. The apparatus includes a tank containing a liquid bath and having ultrasonic energy means for energizing the bath. The ultrasonic means are disposed along one inner vertical wall of the tank.

This application for United States Letters Patent is a continuation-in-part of application Ser. No. 764,849 filed Oct. 3, 1968, now abandoned, which in turn was a continuation-in-part of application Ser. No. 707,299, filed Feb. 21, 1968, now abandoned.

The present invention relates toan apparatus for ultrasonic etching of polymeric printing plates. More particularly, the present invention provides an apparatus including an etching bath energized by ultrasonic energy, a water spray bath for cleaning the etched printing plate, and an air dryer for drying the water Washed printing plate in a current of heated air. In one embodiment, the present invention provides an etching bath energized by means of ultrasonic energy and containing a solvent or aqueous detergent solution, while in another embodiment the present invention provides a solvent or detergent flow system for use in association with the etching bath.

Preparation of relief printing plates made of zinc or magnesium metal by photoengraving requires relatively long etching periods. Recently, polymeric compositions have been substituted for metal in the formation of relief printing plates. However, even with polymeric compositions in which images are formed by photopolymerization or photocuring, the time necessary to develop or etch the plate in a solvent or an aqueous solution borders on that which is commercially acceptable. In addition, it is necessary to also mechanically aid the etching by removal of the unpolymerized or uncured portion of the material with a spray, brush, blotter, sponge or other mechanical means. Such mechanical means, relying on physical contact, cause image surface or edge distortion and the total removal of all the non-image polymeric material is difficult. A further drawback to etching systems for photopolymerizable or photocurable plate materials which use a solvent system is the flammability and toxicity of the solvent.

It has now been found that by the practice of the present invention, an apparatus is provided for etching photopolymerizable or photocurable polymeric printing plates using time cycles of relatively short duration in preparing a polymeric printing plate with a relief image far superior to that obtained by previously used chemical and me chanical methods.

The present invention generally stated provides an apparatus for ultrasonic etching of polymeric printing plates formed of a photocurable or photopolymerizable polymeric composition which has been exposed to actinic light through an image bearing, line or halftone, positive or negative transparency consisting solely of substantially 'ice opaque and substantially transparent areas until substantially complete photocuring or photopolymerization takes place in the exposed areas and substantially no photocuring or photopolymerization takes place in the non-exposed areas. The apparatus includes a tank for containing a liquid bath, ultrasonic energy means disposed along an inner vertical wall of the tank for energizing the bath; means for maintaining the liquid bath at a temperature suflicient to keep non-image areas of the polymeric printing plate in the liquid or molten phase, means for receiving a printing plate, means for receiving a printing plate having exposed and unexposed areas within the tank and for moving the printing plate in an up and down direction during direct cavitation by ultrasonic energy, and means for energizing the bath by ultrasonic energy to a degree necessary to cause cavitation and removal of substantially all of the unexposed non-image area.

An embodiment apparatus of the present invention generally includes an etching bath energized by ultrasonic energy, a water spray bath for cleaning the etched printing plate, and an air dryer for drying the water washed printing plate. Desirably, the etching bath contains a solvent or aqueous detergent solution and a system therefore which continuously monitors the concentration of the solvent or detergent in the etching bath and as required, replenishes depleted solvent or detergent thereby maintaining a desired, substantially constant bath concentration.

Practice of the present invention will become more apparent from the following detailed description taken in conjunction with the drawings wherein:

FIG. 1 illustrates as a perspective view the ultrasonic apparatus of the present invention;

FIG. 2 diagrammatically illustrates the etching bath of FIG. 1 energized by means of ultrasonic energy and having a system associated therewith which continuously monitors the bath concentration and replenishes depleted solvent or detergent as required;

FIG. 3 diagrammatically illustrates a water spray bath for cleaning etched printing plates;

FIG. 4 diagrammatically illustrates an air dryier for drying the water washed printing plates; and

FIG. 5 shows a frame for conveniently mounting a flexible printing plate for use in processing the plate through the several stations of the apparatus of the present invention.

In the drawings wherein similar elements are referred to by similar numerals, FIGS. 1-2 illustrate etching bath tank 10 formed of a suitable material such as metal for retaining a cleaning liquid or etching solution 12 therein.

Disposed relative to tank 10 and desirably along one side vertical wall thereof is a means to energize the bath with ultrasonic energy generated by transducers 14, 16 and 18 positioned within the solution and receiving through lines 20, 22 and 24 respectively and power line 26 electrical energy from amplifier 28 and power supply 30. It is found that more effective etching results by positioning the transducers along the side wall of bath 10 since more direct cavitation is available and materials are readily removed by currents generated in-solution 12. The removed materials may either float to the surface of the solution or may sink to the tank bottom when the plate is substantially vertically disposed therein as illustrated without undue interference with the cavitation of other areas. In effect, therefore, the surface of the printing plate being processed is directly facing the cavitation effect of the transducers.

The ultrasonic activation of the bath to produce cavitation may be provided by various known methods employ ing transducers. The transducers most commonly used for ultrasonic cleaning are either magnetostrictive made of nickel, or its alloys or electrostrictive units made of barium titanate and/ or lead zirconate.

One apparatus for etching printing plates is that illustrated in detail in FIG. 1 wherein a U-shaped frame 13 is oscillated in bath solution 12 while supporting a printing plate being processed therein. The channel 13 is oscillated by arms 15 which are connected for pivotal movement to rod 17. Rod 17 is rotated by horizontal arm 19 which is moved by vertical arm 21 in a to and fro movement indicated by arrow 23, caused by rotation of member 25 on shaft 27. Gear box 29 includes reducing gears to limit the oscillation movement from motor 31.

The transducers on the side of tank 10 are energized at between about 18 to about 40 kilocycles/sec. and preferably about 20 kilocycles/sec. or more and a power density level nominally in the range of about to about 20 watts/ sq. in. or more. It is obvious that longer cleaning or etching periods are required at the lower power density level, i.e. of the order of about 3 to about minutes, and that rapid cleaning of the order of about 0.5 to about 2 minutes may be obtained at the upper power density level for a newspaper page about /2" x 24".

Etching tank 10 may further include in addition to solution 12, a system which continuously montiors the concentration of the solvent or detergent in the etching bath and as required, replenishes depleted solvent or detergent thereby maintaining a desired, substantially constant bath concentration. One useful system may include pipe 32 having a standard valve 34 receiving water from a convenient source for continuously transporting the water at a fixed constant rate to tank 10. Excess solution in tank 10 may be conveniently removed such as by overflow drain tube 36 if desired.

The system may further include a second pipe 38 receiving water from a convenient source for transporting water at a rate regulated by solenoid controlled valve 40 to detergent tank 42 containing a detergent or solution 44 therein. As water is received in detergent tank 42, a corresponding amount of water containing detergent or etching solvent is received by overflow line 46 which transports the overflow to solution 12 in etching tank 10. Control 48 is used to regulate the output through solenoid controlled valve 40 by electrical line 50. The concentration of solution 12 may be monitored by electrode 52 in the etching tank which measures the strength of the solution and transmits the signal to control 48 by electrical line 54. If the concentration of the solution is low, power is supplied to the solenoid valve causing it to open and admit water into the detergent reservoir. Concentration of the solution contained in the reservoir is caused to overflow into the etching tank, thus building up the solution strength. When the strentgh of the solution attains the proper level, the solenoid valve is closed until the strength of the solution again falls below the predetermined level at which the solenoid valve is again opened to correct the depletion.

If desired, heater 56 receiving electrical energy through line 58 may be included in etching tank 10 to maintain the temperature of the solution 12 at a temperature such that the unexposed area of a photocurable or photopolymerizable composition is in the liquid or molten phase. Bath temperatures may range from room temperature up to the boiling point of water at atmospheric pressure and even higher if a pressurized system is maintained. A preferred temperature is in the range of about 150 F. to about 200 F.

Mounting frame 60 described hereinafter in greater detail may be employed to support a photocured polymeric printing plate 62 within solution 12 of etching bath 10. It is found that movement of plate 62 in an up and down direction indicated by arrow 64 further facilitates removal of uncured polymeric composition from the printing plate.

It has been found that by using an aqueous solution of detergent and soap in combination with ultrasonic activation of bath 10 to impart cavitation, the etching time may be reduced to periods of about 5 seconds to about 10 minutes without enlisting mechanical aids such as brushes, sponges, or the like and the relief obtained is far superior in topographical quality and in fidelity of image reproduction to that previously obtained using photopolymerizable or photocurable materials and, in most cases, far superior to the relief formed from a photoengraved metal plate.

The greater the difference in solubility, viscosity, or emulsifiability between the solidified areas in the photosensitized plate and portions of said plate which remain liquid, the greater the effiicency of the etching process to make the relief plate. Thus, it is important to select the right detergent, soap or solvent in the etching bath to facilitate removal by the use of ultrasonics.

The preferred bath for use herein is an aqueous bath containing a suitable, commercially available inorganic or organic detergent in an amount ranging from about 0.05% to about 30% by weight of the aqueous bath.

For the photopolymerizable system described in British Pat. No. 1,102,910, the bath consists of an organic solvent such as acetone or petroleum ether. Etching baths for other liquid photosensitizable compositions are dependent on the difference in solubility between the liquid and solidified portion of the photosensitized relief plate.

Although usually commercially available inorganic or organic detergent compositions may be added to the etching bath, a detergent formed of an alkali metal polyphosphate, an alkali metal silicate, and a linear alkyl benzene sulfonate is preferred to greatly facilitate removal of uncured liquid polymeric composition from selectively photocured printing plates. Although most any alkali meta1 salt may be employed, the sodium salt is preferred.

The amounts of the alkali metal polyphosphate, alkali metal silicate, and linear alkyl benzene sulfonate forming the detergent preferred for use herein may vary provided the amount of the alkali metal silicate comprises a major portion, the amount of the alkali metal polyphosphate comprises a lesser and intermediate portion, and the amount of the linear alkyl benzene sulfonate comprises the minor portion thereof. Desirably, the amount of the alkali metal silicate is greater than the combined amounts of the alkali metal polyphosphate plus the amount of the linear alkyl benzene, sulfonate.

The amount of alkali metal silicate forming the preferred detergent may vary from about 51 to about 97 parts by weight of the detergent. An example of a useful alkali metal silicate is sodium meta-silicate pentahydrate or the like.

The amount of alkali metal polyphosphate forming the preferred detergent may vary from about 2 to about 48 parts by weight of the detergent. An example of a useful alkali metal polyphosphate is sodium tripolyphosphate or the like.

The amount of the linear alkyl benzene sulfonate forming the preferred detergent may vary from about 1 to about 30 parts by weight of the detergent. An example of a useful linear alkyl benzene sulfonate is linear dodecyl benzene sulfonate or the like.

Although the preferred detergent for use herein may be added to form an aqueous bath solution in an amount ranging from about 0.05 to about 30% by weight of the aqueous bath solution, amounts in the range of about 0.5% to about 10% are usually sufiicient and are thus preferred. An amount of about 0.5 by weight has been found to be effective as a concentration for the preferred detergent.

The dwell time in the bath for the printing plate is of the order of about 5 seconds to about 10 minutes, depending upon the photocurable or photopolymerizable material employed, the thickness of the plate, the temperature of the bath, and the cleaning detergent employed in the bath. The relief obtained in such a system constitutes substantially the entire thickness of the photocurable of photopolymerizable layer right down to the support. The printing plate is thereafter removed from the etching bath and passed into water spray rinse tank 66 having a series of spray units 68 disposed along one side with a second series of spray units 70 disposed along an opposite side of the interior of tank 66. The spray units 68 and 70 may be formed of a series of pipes having water spraying elements for directing water under pressure received from water supply 72 against frame 80 and printing plate 82 substantially vertically disposed in the rinse tank. The spray of water may be heated if desired and directed such to wash the printing plate free of any detergent solution remaining thereon as well as any undesired materials including uncured polymeric materials if present. The water wash may be drained from tank 66 by outlet drain 74, or other suitable means as desired.

In order to facilitate rinsing wash water from the surface of printing plate 82, air spraying units 76 and 78 oppositely disposed to receive the plate therebetween may be included to serve to provide an air curtain or air knife which removes substantially most of the water accumulating on the surface of the printing plate as it is removed vertically from the water spraying operation. The air to units 76 and 78 may be heated if desired.

FIG. 4 illustrates air dryer 84 with oppositely disposed air tubes 86 and 88 therein. The air tubes may be used to direct air at elevated temperatures up to about 150 C. to facilitate drying of printing plate 92 in mounting frame 90 when substantially vertically disposed in the air dryer. The air for units 86 and 88 may be received from a pressurized air supply through tube 94 and may be connected to the several units by a manifold arrangement as may be the case for water spray pipes 68 and 70 of FIG. 3 receiving water from supply line 72.

The direction of the air spray or water spray may vary as desired against the printing plate. Variations in the spray pattern may be employed or the spray pattern may be uniform against the printing plate in one or another direction.

FIG. illustrates mount 96 for printing plate 97 to usefully serve as a means for passing the plate through the several processing stations of the present apparatus. It is recognized that other means may be employed, if desired. Mount 96 may appear having a first and a second vertical element 98 conveniently joined to a first and a second horizontal element 100 forming the base frame thereof. Mount handle 102 having L-shaped members 104 slidably projecting through a first horizontal element 100 is biased thereagainst by springs 106 disposed about a longer leg of the L-shaped elements while the perpendicular leg thereof grips plate 97 by holes punched therethrough. The edge opposite the printing plate to that supported by elements 104 may be similarly secured by L-shaped members 108 fixed to the second horizontal element 100 forming the frame.

A convenient method of using the present apparatus is to first place an image-bearing, line or halftone, stencil or positive or negative transparency parallel to the surface of a layer of a photocurable or photopolymerizable composition which has been cast directly on a support. The imagebearing transparency and the surface of the composition may be in contact or have an air gap therebetween, as desired. The photocurable or photopolymerizable layer is exposed through the transparency to a source of actinic light, preferably a point or collimated light source when a liquid photocurable composition is used, until the layer is cured or polymerized to an insoluble stage in the exposed areas.

One photocurable system which is suitable for use herein comprises a polyene defining a liquid polyfunctiona'l component, a liquid polythiol component, and optionally a photocuring rate accelerator.

An example of the polyene component is that represented by the formula wherein m is an integer of at least 2, wherein X is a member selected from the group consisting of:

wherein g is an integer from 1 to 9; R is a radical selected from the group consisting of hydrogen, fluorine, chlorine, furyl, thienyl, pyridyl, phenyl and substituted phenyl, benzyl and substituted benzyl, alkyl and substituted alkyl, alkoxy and substituted alkoxy, and cycloalkyl and substituted cycloalkyl. The substituents on the substituted members are selected from the group consisting of nitro, chloro, fiuoro, acetoxy, acetamide, phenyl, benzyl, alkyl, alkoxy and cycloalkyl. Alkyl and alkoxy have from 1 to 9 carbon atoms and cycloalkyl has from 3 to 8 carbon atoms.

The members (a) to (e) may be connected to [A] through a divalent chemically compatible derivative member of the group consisting of Si(R) carbonate, carboxylate, sulfone,

alkyl and substituted alkyl, cycloalkyl and substituted cycloalkyl, urethane and substituted urethane, urea and substituted urea, amide and substituted amide, amine and substituted amine, and aryl and substituted aryl. The alkyl members have from 1 to 9 carbon atoms, the aryl members are either phenyl or naphthyl, and the cycloalkyl members have from 3 to 8 carbon atoms with R and said members substituted being defined above. B is a member of the group consisting of -O-, --S, and -NR.

The member [A] is polyvalent; free of reactive carbonto-carbon unsaturation; free of highly water-sensitive members; and consisting of atoms selected from the group consisting of carbon, oxygen, nitrogen, chlorine, bromine, fiuorine, phophorus, silicon, and hydrogen,

Another group of polyenes are unsaturated polymers in which double or triple bonds occur primarily within the main chain of the molecules. Examples of this group of polyenes include conventional elastomers such as those derived primarily from standard diene monomers and represented by polyisoprene, polybutadiene, styrene-butadiene rubber, isobutylene-isoprene rubber, polychloroprene, styrene-butadiene-acrylonitrile rubber, and the like; unsaturated polyesters, polyamides, and polyurethanes derived from monomers containing reactive unsaturation and exemplified by adipic acid-butenediol, 1,6-hexanediamine-fumaric acid, 2,4-tolylene diisocyanate-butenediol condensation polymers, and the like.

Another group of polyenes are those in which the reactive unsaturated carbon-to-carbon bonds are conjugated with adjacent unsaturated groupings. Examples of operable reactive conjugated ene systems include but are not limited to the following:

A few typical examples of polymeric polyenes which 7 contain conjugated reactive double bond groupings such as those described above are poly(ethylene ether) glycol (600 M.W.) diacrylate; poly(tetramethylene ether) glycol (1000 M.W.) dimethacrylate; the triacrylate of the reaction product of trimethylol propane with 20 moles of ethylene oxide; diethylene glycol diacrylate; and the like.

The polyene members have a molecular weight in the range from about 64 to 20,000, preferably about 200 to about 10,000; and a viscosity in the range from essentially to 20 million centipoises at 70 C. as measured by a Brookfield viscometer.

The polythiol component of the photocurable polymer composition may be a simple or complex organic compound having a multiplicity of pendant or terminally positioned SH functional groups per average molecule.

The polythiols may be exemplified by the general formula where n is at least 2 and R is a polyvalent organic moiety containing cyclic groupings and hetero atoms such as N, P, or O and primarily containing carbon-carbon, carbon-hydrogen, carbon-oxygen, or silicon-oxygen containing chain linkages free of any reactive carbon-tocarbon unsaturation. On the average the polythiol must contain 2 or more SH groups per molecule and have a viscosity range of essentially 0 to 20 million centipoises (cps) at 70 C. as measured by a Brookfield viscometer either alone or when in the presence of an inert solvent, aqueous dispersant, or plasticizer. Operable polythiols usually have molecular weights in the range about 50 to about 20,000, and preferably from about 100 to about 10,000.

It is possible to adjust the relative amount of polyenes and polythiols to any values above or below the stoichiometric amount which will insolubilize in the imagewise exposed areas and which will give the desirable properties to the cross-linked polythioether. In general, the mole ratio of ene/thiol groups for preparing the curable composition is from about 0.2/1 to about 5/1, and desirably, about 0.75/1 to about 1.5/1 group ratio.

A photocuring rate accelerator may be present as a separate and distinct component of the photocurable composition. The accelerator may be, for example, azobenzene; or a mixture of two or more separate components such as benzophenone, benzanthrone, anthrone, dibenzosuberone, carbon tetrachloride, phenanthrene, and the like; or in a chemically combined form within the molecular structure of either the polyene or the polythiol.

Another composition operable to form a relief printing plate is that set out in British Pats. 1,102,910 and 1,007,- 345. Therein the liquid composition consisting of:

'(A) 0.001 to 5% by weight of a polyboron acid salt of the formula wherein M is a silver (I) or cerium (III) cation, X is halogen; Y is a hydroxyl group, or a hydrocarbyloxyalkoxy or hydrocarbylcarbonyl group free of aliphatic unsaturation and containing at most 12 carbon atoms; n is or 12; p is 1 to 12 and is equal to 11 minus q when q is greater than zero, q is 0 to 2, p+q being at most equal to n; and m is the valence of M; (when p is greater than 1, the halogens represented by X may be the same or different);

(B) A halide promoter in which the halogen is chlorine, bromine or iodine and which is dissociable by actinic light of wavelength between 2500 A.; and 7000 A.; and

(C) At least one substance capable of undergoing cationic polymerization is cationically polymerized on exposure to actinic light having a wavelength in the range 2500 to 7000 A.

Another liquid photocurable composition useful to form printing plates is that set out in French Pat. No.

1,471,432. Generally, liquid photosensitive compositions disclosed in this patent comprise as essential constituents, an unsaturated polyester, an ethylenically unsaturated monomer leading itself to a reaction of addition polymerization, and a photosensitizer. The unsaturated polyester may be produced from an alcohol monomer containing at least a polyol comprising at least 5 etheroxygen radicals linked to carbon atoms in its main chain and not having more than 3 carbon atoms between the ether-oxygen radicals, and a monomer of acidic character containing at least an unsaturated dicarboxylic acid and/ or one of its derivatives.

Thus, in using the present apparatus most any liquid photosensitive composition capable of being photopolymerized or photocured or both upon exposure to actinic radiation to a solid polymer is operable.

The compositions to be converted to printing plates may, if desired, include such additives as antioxidants, accelerators, dyes, inhibitors, activators, fillers, pigments, antistatic agents, flame-retardent agents, thickeners, thixotropic agents, surface active agents, light scattering agents, viscosity modifiers, extending oils, plasticizers, detackifiers and the like, as desired.

The support to which the composition is adhered may be formed from various materials such as rubber, plastic, paper, glass, metal and the like. The support preferably has the characteristics of being flexible, adherable to the photosensitive composition on exposure to actinic radiation or by other means.

The photosensitive reaction may be initiated by actinia radiation from special light sources which emit significant amount of actinic light suitably in the wave length range of 2500-7000 A.; and preferably in the range 3000- 4000 A.

The thickness of the layer of the liquid photosensitive composition employed depends on the thickness desired in the relief image and on the alignment between the relief figures. That is, if the printing areas are closely aligned less relief is necessary than if the printing areas are further apart. This is to assure that the non-printing areas are not contacted with the surface of the material on which the printing is to occur. In the case of photosensitized half-tones formed from the process of the instant invention, the screening used must be taken into consideration when selecting the proper thickness. In general, the thickness of the layer to be solidified and employed as a printing plate may vary from 3 to 250 mils or more. For letterset (dry offset) plates, the thickness may be customarily in the 3 to 25 mil range; for letterpress printing, thicknesses of 10 to 500 mils are common. For letterpress newspaper or magazine printing plates, the thickness of the photosensitive layer will be about 10 to about 50 mils. Thicker layers are somtimes employed for the flexographic printing of designs and relatively large areas with letterpress printing plates.

Printing plates etched by the present apparatus may be employed in for example intaglio printing, lithographic printing, letterpress printing, dry offset printing, flexographic printing and the like.

It is understood that the foregoing detailed description is given merely by way of illustration and that many variations may be made therein without departing from the spirit of this invention.

What is claimed is:

1. An apparatus for etching a printing plate formed of a photocurable polymeric composition, said apparatus comprising, in combination, a tank having a liquid bath therein, ultrasonic energy means for energizing the bath and disposed along one inner vertical wall of the tank, said liquid bath within the tank being maintained at a temperature sufiicient to keep non-image areas of the polymeric printing plate in the liquid or molten phase, means for suspending said printing plate having exposed and unexposed areas within said liquid bath and for oscillating the printing plate in an up and down direction during direct cavitation against the polymeric surface by ultrasonic energy, and means for energizing the liquid bath by ultrasonic energy to a degree necessary to cause cavitation and removal of substantially all of the unexposed non-image area.

2, The apparatus of claim 1 wherein the ultrasonic energy level of the bath varies between about 18 to about 40 kilocycles.

3. The apparatus of claim 1 wherein heating means is provided for heating the liquid disposed in the etching bath.

4. The apparatus of claim 1 wherein water is introduced at a constant rate into the etching bath, liquid is removed from said etching bath by an overflow drain, and means for replenishing depleted detergent concentration in the etching bath.

5. The apparatus of claim 1 wherein a system which continuously monitors the detergent concentration of the liquid is provided in combination with the etching bath, said system including means for replenishing depleted detergent concentration.

the range of about 0.5% to about 10% by weight.

References Cited UNITED STATES PATENTS 1/ 1961 Gutterman. 1341 5/1962 Hightower et a1. 134--1 OTHER REFERENCES Steinberg: Ultrasonics in Industry, p. 1293, publication date precedes file date of parent application 764,849 wherein it was cited.

JACOB H. STEINBERG, Primary Examiner U.S. Cl. X.R. 

